Ferroelectric liquid crystals (FLCs) are potentially useful as the active component in optoelectronic and photonic devices such as optical switches, displays, and spatial light modulators. To be useful in such applications, switching of an FLC, i.e., control of its light-transmitting properties, must be possible. One approach is to dope the liquid crystal with a photochromic compound that produces a reversible photomechanical effect, i.e., a structural change of the liquid crystal phase, in response to irradiation with light of a specific wavelength. The structural change of the liquid crystal phase changes the light-transmitting properties of the liquid crystal. For example, U.S. Pat. No. 5,118,586 to Hattori et al., issued Jun. 2, 1992, proposes the use of a photochromic dopant to achieve such switching in a liquid crystal film, for photorecording of digital information. Irradiation of the aligned liquid crystal film in the ultra-violet (UV) range causes a change in the birefringence of the liquid crystal phase as shown by the rotation of plane-polarized light transmitted through the material. The initial state can be restored by irradiating the film with visible light. As shown by Hattori et al., this principle can be applied to a FLC using a fulgide dopant, in which case the irradiation merely modulates the birefringence of the liquid crystal phase in the absence of an electric field. However, the resulting rotation of plane-polarized light is very small, on the order of 0.2 degree, which is not enough to produce a sufficient contrast in some optoelectronic and photonic applications.
A different approach which may be useful for applications such as optical switches and displays involves a system employing a photoactive chiral dopant in an achiral smectic C(S.sub.C) liquid crystal host (i.e., a liquid crystal phase which exhibits molecular ordering and a molecular tilt) to give an induced FLC. In principle, upon irradiation of the resulting chiral smectic C(S.sub.C *) phase, switching can be achieved via modulation of the spontaneous polarization (P.sub.s ; a chiral bulk property of the FLC) above and below a switching threshold, to change the light-transmitting properties of the FLC. Modulation of P.sub.S is associated with a photo-induced structural change in the dopant.
Recently it has been shown that P.sub.S can be photomodulated in the near-UV range via the reversible trans-cis photoisomerization of chiral azobenzene dopants in S.sub.C or S.sub.C * liquid crystal hosts (Negishi et al., Chem. Lett., 319, 1996; Blinov et al., Jap. J. Appl. Phys., 35:5405, 1996; Sasaki et al., J. Phys. Chem., 99:13013, 1995; Walton et al., Liq. Cryst., 17:333, 1994; Sasaki et al., J. Am. Chem. Soc., 116:625, 1994; Ikeda et al., Nature, 361:428, 1993) and of azobenzene side-chain-doped copolymer FLCs (Oge et al., Macromol. Chem. Phys., 197:1805, 1996). In this system, the modulation of P.sub.S arises from a photomechanical effect associated with the change in shape of the azobenzene dopant from rod-like (trans) to bent (cis): the change to the bent cis-isomer destabilizes the S.sub.C * phase of the liquid crystal and decreases polar ordering, thus decreasing P.sub.S. The photomechanical effect results in significant decreases in the temperature of phase transition between chiral smectic A(S.sub.A *) (i.e., a smectic liquid crystal phase having the same molecular ordering as a S.sub.C * phase, but without a molecular tilt) and S.sub.C * phases or between chiral nematic (N*) (i.e. a liquid crystal phase having orientational ordering only) and S.sub.C * phases, producing in some cases a complete loss of polarization due to an isothermal phase transition to the S.sub.A * or N* phase.
Thus, while the above system has demonstrated photomodulation of P.sub.S, there are significant disadvantages to that approach: (i) the addressing wavelength causing trans-to-cis photoisomerization of the azobenzene dopant is in the near-ultraviolet range (365 nm), which is undesirable for practical applications for which an inexpensive addressing light source is preferred; and (ii) the mechanical destabilization of the S.sub.C * phase can produce a more fluid, less ordered nematic phase with lower structural integrity, which may be a severe impediment if the FLC film is to be used as a long-term digital information storage device.